Refine R2 upload UX with bucket auto-creation attempt and clearer guidance

- Updated `R2Service` to attempt bucket creation via S3 API (`ensureBucketExists`) before upload. This improves the UX for users with Admin keys or custom domains while gracefully failing (with a warning) for users with minimal permissions.
- Modified `UploadModal` helper text to explicitly instruct users about enabling "Public Access" in the Cloudflare dashboard for new buckets.
- Updated `js/r2.js` to ensure the "Public Bucket URL" is always sanitized to include the `https://` protocol, fixing potential webseed generation issues.

components/upload-modal.html

@@ -164,6 +164,9 @@
                             <div class="space-y-1">
                                 <label for="input-r2-domain" class="text-xs font-medium text-text">Public Bucket URL</label>
                                 <input id="input-r2-domain" type="text" class="input text-xs" placeholder="https://pub-xxx.r2.dev" />
+                                <p class="text-[10px] text-muted leading-tight">
+                                    New buckets are private. Enable <strong>Public Access</strong> in the Cloudflare dashboard to get your r2.dev URL.
+                                </p>
                             </div>
                         </div>
                         <div class="grid gap-3 md:grid-cols-2">